This invention relates to an optical amplifying specifically, to an optical amplifying transmission system for optically amplifying and transmitting a wavelength-division-multiplexed light and an optical amplifier therefor.
Because a transmission bandwidth of an optical fiber transmission line is generally wider than an amplifying bandwidth of an optical amplifier, in conventional art, a signal bandwidth is determined by an amplifying bandwidth of an optical amplifier. Furthermore, the interaction between channels such as XPM (Cross Phase Modulation) and FWM(Four Wave Mixing) exist on an optical fiber transmission line causing deterioration of transmission characteristics and, therefore, a channel interval (relative to a wavelength; the following are the same unless noted.) has been conventionally taken wide enough so that the interaction between channels on an optical fiber transmission line can be neglected.
As described above, in conventional art, it is always required to have a wider channel interval. The number of multiplexed wavelengths is determined by the wavelength interval and an amplifying bandwidth of an optical amplifier and, consequently, the primary object of research generally has been aimed at how to widen an amplifying bandwidth of an optical amplifier.
An object of the invention is to provide an optical amplifying transmission system and an optical amplifier which, keeping a wide channel interval, are capable of wavelength-division-multiplexing much more wavelengths.
A further object of the invention is to provide an optical amplifying transmission system and an optical amplifier which can increase the number of multiplexed wavelengths without widening an optical amplifying bandwidth.
In an optical amplifying transmission system according to the invention, a bandwidth compressor for compressing a bandwidth occupied by inputting wavelength-division-multiplexed signal lights and for supplying it to an optical amplifier, is disposed at a front part of said optical amplifier, and a bandwidth expander for expanding the bandwidth occupied by the signal lights output from said optical amplifier is arranged at a rear part of said optical amplifier.
By using the above-mentioned structure, a transmission bandwidth of an optical fiber transmission line and an amplifying bandwidth of an optical amplifier can be determined independently and, as a result, it becomes possible to have a channel interval on an optical fiber transmission line wide enough not to be affected by the interaction between channels, being unrelated to the amplifying bandwidth of the optical amplifier. As the signal lights can be arranged densely within the amplifying bandwidth of the optical amplifier, the number of the multiplexed wavelengths can be increased remarkably and, therefore, the bandwidth of the amplifier can be used effectively. Since there is no need to expand the optical amplifying bandwidth expressly, each signal light can easily obtain a regular gain and, accordingly, gain equalization becomes unnecessary or easy to apply even if necessary. The irregularities of the optical amplifying bandwidths of the optical amplifier can be compensated relatively easily using the wavelength converting function of the bandwidth compressor and the bandwidth expander.
The bandwidth compressor comprises a divider for dividing a inputting wavelength-division-multiplexed signal light into a plurality of groups and a relocator for relocating respective groups of the signal lights divided by the divider, without overlapping each other, within a bandwidth narrower than the original bandwidth and, thus, the bandwidth of the wavelength-division-multiplexed signal light can be compressed by a simple structure.
The relocator comprises a wavelength converter for wavelength-converting each group in a lump except for at least one group in the plurality of groups divided by said divider and a multiplexer for wavelength-multiplexing each group of the signal lights wavelength-converted by the wavelength converter as well as each group of the signal lights which has not wavelength-converted and, therefore, the bandwidths can be compressed effectively using the few wavelength converters.